Silver halide color photography depends on the formation of dyes in order to reproduce an image. These dyes are typically formed from couplers present in or adjacent to the light sensitive silver halide emulsion layers that react to image light upon exposure. During development, the latent image recorded by the silver halide emulsion is developed to amplify the image. During this process in which silver halide is reduced to elemental silver, the color developer compound used is at the same time oxidized, as is typical in a redox reaction. The oxidized developer then reacts or couples with the coupler compound present in or adjacent to the emulsion layer to form a dye of the desired color.
Typically, a silver halide emulsion layer containing a magenta dye-forming coupler is sensitized to green light. This facilitates so-called negative-positive processing in which the image is initially captured in a negative format where black is captured as white, white as black, and the colors as their complimentary colors (e.g., green as magenta, blue as yellow, and red as cyan). Then the initial image can be optically printed in the correct colors through the device of optical printing which has the effect of producing a negative of the negative, or a positive image of the original scene.
For incorporation into a photographic element, the couplers are typically dissolved in high-boiling organic solvents known as "coupler solvents," and dispersed in gelatin with the aid of surfactants.
The present invention is concerned with the preparation of 4H-pyrazolo1,5-a!benzimidazole compounds useful as magenta dye-forming couplers (hereinafter referred to as "PBI" compounds or couplers). These compounds may broadly be described by Formula (A) ##STR1## in which R.sub.2 and R.sub.5 -R.sub.8 represent substituents and Z represents a hydrogen atom or a group capable of being split off during the coupling reaction. German patent 1,070,030 discloses PBI couplers that form magenta dyes upon coupling. In the examples given, R.sub.2 represents an alkyl or phenyl group. Couplers of these types have been found to have poor coupling reactivity, to yield image dyes whose absorption spectra are too bathochromic for practical use in color photographic papers, and to have poor stability to light. International Patent Application WO 91/14970 describes PBI couplers with specifically substituted alkylthio coupling-off groups, including carboxyalkylthio groups. Such couplers offer marked improvements in coupling reactivity but do not offer improved dye hue or light stability. U.S. Pat. No. 5,143,821 describes PBI couplers in which R.sub.2 represents an alkoxy group. Such couplers are advantageous because they have much better coupling reactivity than those in which R.sub.2 represents an alkyl group, and the image dyes formed from them have good spectral absorption characteristics. Moreover, the dyes from these couplers have better light stability than the dyes from PBI couplers in which R.sub.2 is an alkyl group. However, the tendency to yellow of these alkoxy PBI couplers is unacceptable for the formation of accurate image reproductions, especially for color photographic papers.
U.S. Pat. Nos. 4,241,168 and 4,076,533 suggest the possible use of azole coupling-off groups on magenta couplers. As these patents teach, the standard procedure for introducing the N heterocycles typically involves halogenation of the coupling site followed by displacement with the N-heterocycle. Unfortunately, in the case of the PBI compounds, the halogen derivatives are unreactive toward nitrogen heterocycles at room temperature. Heating the reaction results in the thermal disproportionation of the halogen derivative that renders the yield of the desired product unsatisfactory. The use of a stabilizer such as .beta.-hydroxytoluene (BHT), does not appear to inhibit this disproportionation, and thus this route is unfeasible.
It is a problem to be solved to provide a step for synthesizing intermediates useful for appending a N heterocyclic compound to the 3-position of a 4-H-pyrazolo-1,5-a!benzimidazole compound and a combined process that yields the target compounds in good yields while using inexpensive reagents and mild conditions.